Challenging ICES age estimation protocols: lessons learned from the eastern Baltic cod stock

Abstract

Over the recent decades, the International Council for the Exploration of the Sea (ICES) has set guidelines for best practise quality control of age estimation procedures. The applicability of these guidelines is assessed by reviewing the ageing issues of eastern Baltic cod (EBC) as a case study. Since the implementation of an age-based assessment of EBC in the beginning of the 1970s, the assessment has been hampered by the quality of the age composition data, in recent years to a degree that age-based assessment is no longer used. The reason for the age reading problems is the low visual contrast between growth zones in the otoliths which seems to be the result of complex interactions of the hydrography in the Baltic Sea with the cod’s biology and behaviour. Over the last 40 years, various expert groups have struggled to document and improve the agreement of age estimation between national otolith readers, standardize methods and age estimations through repeated exchanges and reference collections as well as an internationally agreed manual. Despite these initiatives the precision of the age estimations based on traditional ageing did not improve, with significant bias persisting between and within readers. Additionally, a wide range of alternative methods for deriving the age information necessary for stock assessment and for validation of the true age have been tested. However, these methods did not produce unbiased age estimates over the entire size and age range of the EBC stock. An age-validation is urgently needed. Deviations from the ICES guidelines identified are as follows: (i) the lack of rigorous quality control, particularly the auditing of national trends in age precision over the years using a reference collection and (ii) the implementation of an age error matrix in the stock assessment.

How old are you-Evaluation of age reading methods for the invasive round goby (Neogobius melanostomus , Pallas 1814)

“…Two of the commercially most important fish species in the Baltic Sea, flounder and cod, (Gadus morhua Linnaeus 1758) suffer from problems with the quality of age readings (Hüssy et al, 2016;ICES, 2014), suggesting that age determination of demersal fish in the Baltic Sea might be especially problematic.…”

“…This indicates significant reader related bias and highlights the need for calibration exercises among readers in order to standardise age reading practices. Two of the commercially most important fish species in the Baltic Sea, flounder and cod, ( Gadus morhua Linnaeus 1758) suffer from problems with the quality of age readings (Hüssy et al., ; ICES, ), suggesting that age determination of demersal fish in the Baltic Sea might be especially problematic.…”

Effect of Marine Hypoxia on Baltic Sea Cod Gadus morhua: Evidence From Otolith Chemical Proxies

“…Part of the hearing and balance system in modern fishes, otoliths grow as the fish grows, providing a unique history for each individual (Campana and Neilson, 1985); and over 8 × 10 5 otoliths are aged annually for stock assessments by major fisheries labs around the world (Campana and Thorrold, 2001). Baltic cod otoliths, always somewhat difficult to age (Hüssy, 2010), became increasingly unreadable through the 2000s (Hüssy et al, 2016). In 2014, the stock assessment failed, due in great part to large uncertainties and inconsistencies in age determination (International Council for the Exploration of the Seas [ICES], 2014;Eero et al, 2015;Hüssy et al, 2016).…”

“…Baltic cod otoliths, always somewhat difficult to age (Hüssy, 2010), became increasingly unreadable through the 2000s (Hüssy et al, 2016). In 2014, the stock assessment failed, due in great part to large uncertainties and inconsistencies in age determination (International Council for the Exploration of the Seas [ICES], 2014;Eero et al, 2015;Hüssy et al, 2016).…”

Fish and Sclerochronology Research in the Mediterranean: Challenges and Opportunities for Reconstructing Environmental Changes

“…Clarity of growth rings in otoliths is one of the main factors contributing to sclerochronology research, both for growth increment measurements and for otolith chemical analysis (Campana, 1999). Problems related to interpretation of increments in otolith, including age estimation and validation of periodicity, has been pointed out in number of studies in different parts of the world (e.g., Morales-Nin et al, 2005b;Stransky et al, 2005;Hüssy et al, 2016). This problem should not be underestimated, and interpretation of otolith increments needs to be carefully checked and validated.…”